Method for pressure modulation of turbine sidewall cavities

ABSTRACT

A method is provided for controlling cooling air flow for pressure modulation of turbine components, such as the turbine outer sidewall cavities. The pressure at which cooling and purge air is supplied to the turbine outer side wall cavities is modulated, based on compressor discharge pressure (Pcd), thereby to generally maintain the back flow margin (BFM) so as to minimize excessive leakage and the consequent performance deterioration. In an exemplary embodiment, the air pressure within the third stage outer side wall cavity and the air pressure within the fourth stage outer side wall cavity are each controlled to a respective value that is a respective prescribed percentage of the concurrent compressor discharge pressure. The prescribed percentage may be determined from a ratio of the respective outer side wall pressure to compressor discharge pressure at Cold Day Turn Down (CDTD) required to provide a prescribed back flow margin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 09/491,599, filed Jan. 25, 2000, now pending, the entire content of which is hereby incorporated by reference in this application.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuant to Contract No. DE-FC21-95MC31176 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

The invention relates to cooling and purge air-flow systems for gas turbines.

A gas turbine engine conventionally includes a compressor for compressing ambient air for being mixed with fuel and ignited to generate combustion gases in a combustor. A turbine receives the hot combustion gases and extracts energy therefrom for powering the combustor and producing output power, for example for powering an electrical generator. The turbine conventionally includes one or more stages of stator nozzles or vanes, rotor blades and annular shrouds around the turbine blades for maintaining appropriate clearances therewith. As turbine inlet temperatures have increased to improve the efficiency of gas turbine engines, it has become necessary to provide a cooling fluid such as air to the turbine vanes, blades and shrouds to maintain the temperatures of those components at levels that can be withstood by the materials thereof, to ensure a satisfactory useful life of the components. Cooling is typically accomplished by extracting a portion of the air compressed by the compressor from the compressor and conducting it to the components of the turbine to cool the same. Any air compressed in the compressor and not used in generating combustion gases necessarily reduces the efficiency of the engine. Therefore, it is desirable to minimize the amount of cooling air bled from the compressor. Furthermore, the air used for cooling turbine components typically discharges from orifices or gaps in those components. That cooling air mixes with the combustion gases in the turbine and will also reduce engine efficiency for thermodynamic and aerodynamic reasons. Accordingly, while turbine efficiency increases as turbine inlet temperature increases, that increase in temperature also requires effective cooling of the heated components, and such cooling is optimally effected in a manner so as not to forfeit the increased efficiency realized by the increased temperature.

The axial location or stage where the air is bled from the compressor is determined by the pressure required by the component or system to be serviced by that air. To ensure sufficiently high delivery pressure, in general it is desirable to select the source with the highest possible pressure. However, bleeding air from the earliest possible stage of the compressor will increase compressor efficiency by reducing the amount of work invested in the extracted air. Therefore, it is desirable to achieve the highest possible system supply pressure from the earliest and lowest pressure stage of the compressor.

Furthermore, the cooling air must be provided at suitable pressures and flow rates to not only adequately cool the turbine component(s) but to maintain acceptable back flow margin(s). Back flow margin is defined as the difference between the cooling air pressure inside, for example, the outer side wall, and the pressure of the hot combustion gases which flow through the turbine. A positive back flow margin is desirably maintained so that combustion gases are not ingested into the outer sidewall.

BRIEF SUMMARY OF THE INVENTION

The pressure in the outer side wall cavities of the third and fourth stage nozzles is critical for setting back flow margin (BFM). As noted above, the purpose of BFM is to stop ingestion of hot flow-path gas into the outer side wall cavities. BFM is the percentage that the cavity pressure is above the maximum pressure in the associated gas path. Usually, for a nozzle stage, the maximum pressure will be the maximum stagnation pressure on the airfoil lead edge. Conventionally, the minimum back flow margin (e.g. 3%) is set according to the worst conditions to which the component is to be exposed. In the case of some gas turbine systems, worst conditions occur at Cold Day Turn Down (CDTD), typically about 0-degrees and half power. Under these conditions, maximal compressor bleed flow is appropriate because the compressor bleed pressure is low compared to the compressor outlet pressure. When the BFM is set at CDTD, however, the actual back flow margin will increase away from that at CDTD because under other conditions, the bleed pressure from the compressor is higher than at CDTD. If the BFM is too large, the outer side cavities will leak excessively and as a result combined cycle performance will deteriorate. In that regard, as noted above, a negative consequence of this excessive leakage is a performance loss because the work/power used to compress that air is wasted.

To balance these two opposing requirements, the invention is embodied in a cooling and purge air supply system wherein the pressures in the outer side wall cavities are modulated, based on compressor discharge pressure (Pcd) in the presently preferred embodiment, thereby to generally maintain the BFM so as to minimize excessive leakage and the consequent performance deterioration.

Thus, the invention may be embodied in a cooling air flow control system for a gas turbine. In such an embodiment, the cooling air flow control system comprises a valve that controls the pressure of cooling air conducted from the compressor to the turbine and a controller for controlling the valve in accordance with an operating condition of the gas turbine, for example, the compressor discharge pressure. In an exemplary embodiment, during at least a portion of the operating range of the gas turbine, the control system controls the valve to substantially maintain the air pressure within a component of the turbine at a value that is a prescribed percentage of the concurrent compressor discharge pressure.

The invention may also be embodied in a system and method for providing cooling and/or purge air from a multi-stage compressor to an associated turbine for cooling at least one component of the turbine and/or preventing back flow of combustion gases thereinto wherein the pressure of the cooling and/or purge air is controlled according to an operating condition of the turbine, for example, the compressor discharge pressure. In an exemplary embodiment, the pressure of the cooling and/or purge air is controlled to substantially maintain the air pressure within the component of the turbine at a value that is a prescribed percentage of the concurrent compressor discharge pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine having pressure modulation for the third and fourth stage outer side wall cavities in an embodiment of the invention;

FIG. 2 is a plot of Posw/Pcd versus Pcd and BFM versus Pcd for the stage three outer side wall cavity; and

FIG. 3 is a plot of Posw/Pcd versus Pcd and BFM versus Pcd for the stage four outer side wall cavity.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated schematically in FIG. 1 is an exemplary gas turbine used for powering, for example, an electric generator (not shown). The gas turbine includes compressor 10 having an annular inlet 12 for receiving and compressing air, which is discharged from an annular outlet 14 of the compressor 10 at maximum pressure. The compressor is an axial compressor having alternating rows of stator vanes and rotor blades arranged in a plurality of stages for sequentially compressing the air, with each succeeding downstream stage increasing the pressure higher and higher until the air is discharged from the compressor outlet at maximum pressure, hereinafter the compressor discharge pressure Pcd. A conventional combustor 16 receives the compressed outlet air from the compressor outlet 14. Conventional fuel supply conduits and injectors (not shown) are further provided for mixing a suitable fuel with the compressed outlet air for undergoing combustion in the combustor, to generate hot combustion gases. Disposed downstream from the combustor 16, in flow communication therewith, is a turbine section 18 where the energy of the hot gases is converted into work. This conversion takes place in two steps. The hot gases are expanded and the portion of the thermal energy is converted into kinetic energy in the nozzle section of the turbine. Then, in the bucket section of the turbine a portion of the kinetic energy is transferred to the rotating buckets and converted to work. A portion of the work developed by the turbine is used to drive the compressor whereas the remainder is available for generating power. The turbine thus has various components that are heated by the combustion gases, including the respective rows of stator nozzles or vanes and rotor blades that are arranged in a plurality of stages therein. As mentioned above, cooling air must be provided at suitable pressures and flow rates to cool these turbine components and to maintain an acceptable back flow margin.

Efficient operation of the gas turbine requires that a number of critical turbine operating parameters be processed to determine optimal settings for controllable parameters such as fuel flow and distribution and intake air-flow. Such operating parameters include compressor inlet and outlet temperatures and pressures, exhaust temperature and pressure and the like.

An example of a control system for a gas turbine is the General Electric Co.'s Speedtronic™ Mark V Control System, which is designed to fulfill all gas turbine control requirements, including speed and load control functions which act to control fuel flow under part-load conditions and temperature control which limits fuel flow to a maximum consistent with achieving rated firing temperatures, and controls air-flow via the inlet guide vanes. The Mark V control system also handles sequencing of the auxiliaries to allow fully automated start-up, shut-down and cool-down. Incorporated in the basic system are turbine protection against adverse operating conditions and annunciation of abnormal conditions.

Thus, the control system performs many functions including fuel, air and emissions control, sequencing of turbine fuel and auxiliary for start-up, shut-down and cool-down, synchronization and voltage matching of generator and system, monitoring of all turbine, control and auxiliary functions, and protection against unsafe and adverse operating conditions. All of these functions are performed in an integrated manner to perform the desired preprogrammed and/or operator input control philosophy. An exemplary control system of the type summarized above is described in greater detail in U.S. Pat. No. 5,857,321, the disclosure of which is incorporated herein by this reference.

As mentioned above, the invention is embodied in a cooling and purge air supply system wherein the pressures in the outer side wall cavities are controlled/modulated by the control system, so that desired cooling is provided and back flow margin is maintained while minimizing excessive leakage. In the presently preferred embodiment, the outer side wall cavity pressures are controlled to be a prescribed percentage of compressor discharge pressure (Pcd).

Thus, referring to FIG. 1, the present invention provides cooling air path(s) for conducting compressed air bled from or extracted from the compressor (interstage extraction) to selected component(s) of the turbine. For ease of illustration, the cooling circuits shown are substantially limited to those for cooling the third and fourth stage outer side wall and maintaining the pressure thereof sufficiently high to prevent back flow of hot combustion gases into those outer side wall cavities.

More specifically, the illustrated cooling circuits include extraction conduits or lines 20, 22, 24, 26 disposed in flow communication with an extraction point or stage of the compressor 10 for bleeding a portion of the compressed air as cooling or bleed air at a corresponding extraction pressure. Compressed air bleed lines can extend to any one of the heated components in the turbine requiring cooling and/or purge air. In the illustrated embodiment, the compressor air extraction conduits 20, 22, 24, 26 are connected to a bleed air extraction manifold that surrounds e.g., the twelfth or thirteen stage of the compressor section and directs the cooling air to the third and fourth stages of the turbine section 18. Line 28 conducts the compressor bleed air from conduits 20 and 22 to branch conduits 32. Branch conduit 32 flows air, via a flow restrictor, an orifice plate 38 in this example, to the third and fourth stage outer wall cavities, to provide a minimum requirement of cooling and purge air, as discussed in greater detail below.

Line 40 conducts the compressor bleed air from conduits 24 and 26 to branch conduit 42 for flowing air, via a variable orifice valve 44 to modulate the pressure of the air fed to the target outer side wall cavity(ies) in accordance with an operating condition of the gas turbine, such as a detected compressor discharge pressure. The variable orifice valve 44 is preferably designed to define a minimum orifice or operated to define a minimum orifice for Full Speed Full Load (FSFL) operation and designed to define or operated to define a maximum flow orifice or full open for CDTD operation. For compressor discharge pressure below CDTD, the mechanism is preferably adapted to remain open and for Pcd above FSFL, the mechanism is preferably adapted remain at minimum open. The variable orifice valve is otherwise suitably controlled for determining the valve opening in accordance with operating conditions, as discussed in greater detail below.

The compressor bleed air is conducted through branch conduit 42 from the variable orifice valve 44 to the outer side wall at 46 where the supplied cooling and purge air is fed internally for cooling the outer side wall. The air supplied to the outer side wall cavity is vented into the gas path via openings (not shown), e.g. in the trailing edge of the third stage nozzle 36.

In the system and method provided according an exemplary embodiment of the invention, cavity pressure Posw and potentially temperature are sensed with suitable disposed sensors schematically shown at 50 as is the compressor discharge pressure Pcd, e.g. with a suitable sensor 48, and the valve orifice 44 is controlled to determine the outer wall cavity pressure. More specifically, the pressure of the stage three nozzle outer side wall cavity and the pressure of the stage four nozzle outer side wall cavity are measured/monitored and compressed air flow thereto via the variable orifice valve 44 is controlled to maintain the pressures of these cavities within prescribed limits, for example, to a prescribed percentage of the compressor discharge pressure. Thus, in the presently preferred embodiment, as described in greater detail below, the pressure in the outer side wall cavity of the third stage nozzle is set to about 28.2% of compressor discharge pressure Pcd over the relevant operation range (Turn Down [TD] to Full Load, and from Cold Day to Hot Day). The pressure, and temperature, of the third stage nozzle outer side wall cavity is preferably measured using triple redundancy with the sensors ideally being located in the lower half of the outer casing. Also according to the presently preferred embodiment, as described in greater detail below, the pressure in the outer side wall cavity of the fourth stage nozzle is set to about 9.8% of compressor discharge pressure Pcd over the relevant operation range (TD to Full Load, and from Cold Day to Hot Day). The pressure of the fourth stage outer sidewall cavity is preferably measured using double redundancy and, again, the location of the sensors is ideally in the lower half of the outer casing.

The system is also advantageously designed so as that in the event of component failure, that failure will not lead to catastrophic turbine failure. As parts life and operation is dependent upon component cooling and preventing back flow of hot combustion gases, the flow control mechanism 44 provided as an embodiment of the invention advantageously fails open and/or a parallel flow path is provided to supply cooling and purge air flow in the event of valve 44 failure. Thus, in the illustrated embodiment, branch conduit 32 is provided as a second flow path for compressor bleed air. As described above, branch conduit 32 includes a flow restrictor, specifically an orifice plate 38, for defining a minimum flow of compressor bleed air to the turbine 18 in the event of variable orifice 44 failure. Although in the presently preferred embodiment, an orifice plate is provided, it is to be understood that another similar such component may be provided to enable the appropriate fail-safe conditions.

As the gas turbine operates off design, away from full speed full load (FSFL) the work distribution in the compressor will change and so will the relationship between the interstage pressures. Accordingly, a spreadsheet was set up to find an appropriate rule for controlling outer sidewall cavity pressure. More particularly, four steady state cycle points were examined, namely Cold Day Turn Down (CDTD); ISO Turn Down (ISO-TD), Full Speed Full Load (FSFL) and Cold Day (CD). In this example, the cooling/purge air was extracted from the twelfth (12^(th)) stage of the compressor. It is to be understood, however, that this invention is not to be limited to a particular extraction source. The data is summarized in TABLE 1.

TABLE 1 Third and Fourth Stage Outer Side Wall Cavities ISO Case CDTD FSFL Cold Day turn down P12 80.5 147.5 154.63 102.56 Pcd 224.07 338.11 382.93 252.77 P3-nozzlemax-estimate 59.583 89.2 101.2213 66.99 P4-nozzlemax-estimate 21.1765 29.7 34.2212 23.2624 P12/Pcd 0.3593 0.4362 0.4038 0.4057 [P12/Pcd]/[P12/Pcd]fsfl 0.8235 1 0.9256 0.9301 P3osw/Pcd-limits 0.282 0.282 P4osw/Pcd-limits 0.098 0.098 P3osw/Pcd-desired 0.282 0.282 0.282 0.282 P4osw/Pcd-desired 0.098 0.098 0.098 0.098 P3osw-desired 63.1877 95.347 107.9863 71.2811 P4osw-desired 21.9589 33.1348 37.5271 24.7715 P3osw/P12-desired 0.7849 0.6464 0.6984 0.695 P4osw/P12-desired 0.2728 0.2246 0.2427 0.2415 BFM 3-desired 6.0499 6.8913 6.6833 6.4056 BFM 4-desired 3.6946 11.5649 9.6606 6.4873

As will be appreciated, the relation [P12/Pcd]/[P/12/Pcd]fsfl shows the degradation in Stage 12 pressure for off design conditions.

Referring to FIGS. 2 and 3, actual and desired values for the outer sidewall pressure (Posw/Pcd) are plotted versus Pcd and actual and desired values for the back flow margin BFM are plotted versus Pcd. The actual values depicted in FIGS. 2 and 3 demonstrate that the cavity pressure and thus the back flow margin increase away from the threshold established for the worst conditions. This means that the cavity pressure, and thus the back flow margin is greater than necessary to preclude back flow and results in excessive leakage. It would thus be desirable first for the flow of cooling air bled from the compressor to be sufficient for the part to be sufficiently cooled to optimize component life without having been excessively cooled (which wastes the work invested in the cooling air) and, second, for the pressure of the air to be sufficient for back flow to be precluded.

The invention is thus embodied in a gas turbine system wherein the pressure in the outer side wall cavities is modulated to maintain the outer side wall cavity pressure at a level sufficient to satisfy the requirement for a minimum BFM under the worst conditions and to ensure sufficient pressure and cooling flow, while minimizing excessive leakage and cooling. In an exemplary embodiment, pressure in the outer side wall cavities is modulated to be maintained at substantially a constant percentage of the compressor discharge pressure. In the presently preferred embodiment, that prescribed percentage corresponds to a ratio of the component air pressure, in this case the outer side wall cavity, to compressor discharge pressure required at Cold Day Turn Down (CDTD) to provide the prescribed minimum back flow margin.

Thus, in an exemplary embodiment, the gas turbine and air flow passages are adapted to set the pressure of the third (3^(rd)) stage outer side wall cavity to a predetermined ratio of approximately 28.2% of the compressor discharge pressure. The turbine control system is thus adapted to detect the pressure present in the third stage outer side wall cavity and the compressor discharge pressure, and to address compressor bleed to generally maintain the predetermined ratio of third stage outer side wall cavity pressure to compressor discharge pressure. The maximal back flow margin is correspondingly maintained near the minimum defined by the CDTD so as to minimize waste due to excessive leakage. Thus, referring to TABLE 1, it can be seen that in the case of Cold Day Turn Down, with a prescribed and controlled ratio of third stage outer side wall pressure to compressor discharge pressure, where the compressor discharge pressure is 224.07 and the pressure of compressor stage twelve air is 80.5, the third stage outer side wall pressure would be controlled, in this example, to approximately 63.19, i.e. 28.2% of the compressor discharge pressure. Meanwhile, the estimated maximum third stage nozzle pressure under these conditions is 59.58, such that the back flow margin, the percentage cavity pressure is above the maximum pressure in the gas path, would be approximately 6.05%.

In the case of Full Speed Full Load, it is seen that the pressure of the compressor stage twelve air is increased to 147.5 and the compressor discharge pressure is increased to 338.11. With the pressure of the third stage outer side wall controlled to about 28.2% of the compressor discharge pressure, in this exemplary embodiment, the third stage outer side wall cavity pressure would be maintained at about 95.35. Since under these conditions the maximum nozzle pressure is estimated to be 89.2, it is seen that the back flow margin would be maintained to approximately 6.89%. Similarly in the case of Cold Day conditions and ISO turndown, controlling the pressure of the third stage outer side wall cavity to a prescribed percentage of the compressor discharge pressure, about 28.2% in this example, the back flow margin will be controlled to be maintained generally near the threshold established for the worst conditions (CDTD), thereby to minimize excessive leakage.

In a presently preferred embodiment of the invention, the target fourth (4^(th)) stage outer side wall cavity pressure is also modulated to maintain the fourth stage outer side wall cavity pressure to satisfy the requirement for a minimum BFM under the worst conditions, to ensure sufficient pressure and cooling flow, while minimizing excessive leakage and cooling. In an exemplary embodiment, as noted above, pressure in the outer side wall cavities is modulated to be maintained at substantially a constant percentage of the compressor discharge pressure, and in the presently preferred embodiment, that prescribed percentage corresponds to a ratio of the component air pressure, in this case the outer side wall cavity, to compressor discharge pressure required at Cold Day Turn Down (CDTD) to provide the prescribed minimum back flow margin.

Thus, in an exemplary embodiment, the gas turbine and air flow passages are adapted to set the pressure of the fourth (4^(th)) stage outer side wall cavity to a predetermined ratio of approximately 9.8% of the compressor discharge pressure.

By maintaining the ratio of the fourth stage outer side wall cavity pressure to the compressor discharge pressure at a prescribed value, in the presently preferred embodiment generally corresponding to the ratio required at CDTD to provide the prescribed minimum back flow margin, throughout turbine operation, a back flow margin is provided that is sufficient to purge, but avoiding the excessive leakage seen with the conventional system, illustrated by the actual values in FIG. 3.

The turbine control system is thus also adapted to detect the pressure present in the fourth stage outer side wall cavity, and to adjust compressor bleed to maintain the predetermined ratio of fourth stage outer side wall cavity pressure to compressor discharge pressure. The maximal back flow margin is correspondingly maintained generally near the minimum defined by CDTD so as to minimize waste due to excessive leakage. Thus, referring to TABLE 1, it can be seen that in the case of Cold Day Turn Down, with a prescribed and controlled ratio of fourth stage outer side wall pressure to compressor discharge pressure, where the compressor discharge pressure is 224.07 and the pressure of compressor stage twelve air is 80.5, the fourth stage outer side wall pressure will be controlled, in this example, to approximately 21.9589, i.e. 9.8% of the compressor discharge pressure. Meanwhile, the estimated maximum fourth stage nozzle pressure under these conditions is 21.1765, such that the back flow margin, the percentage cavity pressure is above the maximum pressure in the gas path, will be approximately 3.69%.

In the case of Full Speed Full Load, it is seen that the pressure of the compressor stage twelve air is increased to 147.5 and the compressor discharge pressure is increased to 338.11. With the pressure of the fourth stage outer side wall controlled to 9.8% of the compressor discharge pressure, in this exemplary embodiment, the fourth stage outer side wall cavity pressure will be maintained at about 33.1. Since under these conditions the maximum fourth stage nozzle pressure is estimated to be 29.7, it is seen that the back flow margin will be approximately 11.6%. While this is some three times the BFM for CDTD conditions, it is still only about one third of the margin seen in the conventional system, illustrated by the actual value plot in FIG. 3. Similarly in the case of Cold Day conditions and ISO turndown, controlling the pressure of the fourth stage outer side wall cavity to a prescribed percentage of the compressor discharge pressure, about 9.8% in this example, the back flow margin is controlled to be maintained substantially closer to the threshold established for the worst conditions (CDTD) than the conventional system, thereby to minimize excessive leakage.

To control P3osw and P4osw according to an operating condition of the turbine, for example to respective prescribed percentages of the Pcd, two control valves 44 can be provided, one for the third stage outer side wall and one for the fourth stage outer side wall. However, in the illustrated embodiment, only a single air supply 42 and control valve 44 is shown for controlling flow to the third stage outer side wall. To control the flow to/pressure in the fourth stage outer side wall in this embodiment, then, there are holes (not shown) from the third stage to the fourth stage which are suitably determined to insure that the fourth stage outer side wall has the correct amount of purge flow per its pressure (9.8% of Pcd in the presently preferred embodiment).

As is evident from the foregoing, to facilitate implementation of a prescribed pressure modulation, a relationship between outer side wall pressure and compressor discharge pressure has been selected corresponding to that required at Cold Day Turn Down (CDTD) to provide the prescribed minimum back flow margin and is applied during at least a portion of a range of turbine operations, e.g. TD to Full Load, and from Cold Day to Hot Day. It is to be understood, however, that the pressure modulation is not limited to the disclosed example, so long as sufficient cooling flow can be realized and sufficient back flow margin for continuous purging is provided.

In that regard, the back flow margin is conventionally set, as noted above, according to the worst conditions and further is set to allow a sufficient buffer to accommodate machine to machine variations. Furthermore, different manufactures will determine appropriate pressures and margins for optimizing net efficiency and net output. Thus, for example, the desired ratio of outer side wall pressure to compressor discharge pressure, and consequently back flow margin, may be not only machine dependent but also manufacturer dependent.

Accordingly, while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

What is claimed is:
 1. A method for providing cooling and/or purge air from a multi-stage compressor to an associated turbine for cooling at least one target component of said turbine and/or preventing back flow of combustion gases thereinto comprising: selectively extracting bleed air from a stage of the compressor that is upstream of a discharge of the compressor, flowing at least a portion of said extraction through a first flow path from said compressor towards a first target component of said turbine; and controlling flow along said flow path to selectively control an air pressure at said first target component of said turbine according to an operating condition of at least one of said compressor and said turbine.
 2. A method as in claim 1, comprising controlling said air pressure, during at least a portion of an operating range of said turbine, according to a discharge pressure of said compressor section.
 3. A method as in claim 2, wherein said flowing step comprises flowing said cooling and/or purge air to an interior of said first target component, and wherein said controlling step comprises controlling said air pressure to substantially maintain the air pressure within said component at a value that is a prescribed percentage of a discharge pressure of said compressor.
 4. A method as in claim 3, wherein said prescribed percentage corresponds to a ratio of the component air pressure to compressor discharge pressure at Cold Day Turn Down (CDTD) required to provide a prescribed difference between the component air pressure and a pressure of combustion gases flowing through the turbine.
 5. A method as in claim 1, further comprising flowing bleed air along a second flow path from said compressor towards said first target component of said turbine, at least a portion of said second flow path being disposed in parallel to said first flow path.
 6. A method as in claim 5, further comprising controlling flow along said second flow path to control a pressure of air delivered therethrough to said first target component.
 7. A method for providing cooling and/or purge air from a multi-stage compressor to an associated turbine for cooling at least one target component of said turbine and/or preventing back flow of combustion gases thereinto comprising: selectively extracting bleed air from a stage of the compressor, flowing at least a portion of said extraction through a first flow path from said compressor towards a first target component of said turbine; controlling flow along said flow path to selectively control an air pressure at said first target component of said turbine according to an operating condition of at least one of said compressor and said turbine, and controlling said air pressure, during at least a portion of an operating range of said turbine, according to a discharge pressure of said compressor section, wherein said flowing step comprises flowing said cooling and/or purge air to an interior of said first target component, and wherein said controlling step comprises controlling said air pressure to substantially maintain the air pressure within said component at a value that is a prescribed percentage of a discharge pressure of said compressor.
 8. A method as in claim 7, wherein said prescribed percentage corresponds to a ratio of the component air pressure to compressor discharge pressure at Cold Day Turn Down (CDTD) required to provide a prescribed difference between the component air pressure and a pressure of combustion gases flowing through the turbine. 